High solids dissolved air flotation system and methods

ABSTRACT

A wastewater treatment system including an aeration unit, a contact tank, a dissolved air flotation unit, and a biological treatment unit is disclosed. A method of retrofitting a wastewater treatment system by providing an aeration unit and fluidly connecting the aeration unit to the wastewater treatment system is also disclosed. A method of treating wastewater including aerating wastewater with oxygen, combining the aerated wastewater with activated sludge, floating biosolids from the activated wastewater, and biologically treating the effluent is also disclosed. The method optionally includes combining the floated biosolids with the aerated wastewater and/or activated wastewater. A method of facilitating treatment of high solids content wastewater is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119 of U.S. PatentApplication No. 62/642,632, titled “High Solids Captivator Process,”filed on Mar. 14, 2018, which is incorporated herein by reference in itsentirety for all purposes.

FIELD OF TECHNOLOGY

Aspects and embodiments disclosed herein are directed toward systems andmethods for the treatment of wastewater.

SUMMARY

In accordance with an aspect, there is provided a wastewater treatmentsystem. The wastewater treatment system may comprise an aeration unit, acontact tank, a dissolved air flotation unit, and a biological treatmentunit. The aeration unit may have a first inlet fluidly connectable to asource of a wastewater to be treated, a second inlet fluidly connectableto a source of oxygen, a third inlet, and an outlet. The aeration unitmay be configured to aerate the wastewater with the oxygen to form afirst mixed liquor. The contact tank may have a first inlet fluidlyconnected to the outlet of the aeration unit, a second inlet, and anoutlet. The contact tank may be configured to treat the first mixedliquor with activated sludge to form a second mixed liquor. Thedissolved air flotation unit may have a first inlet fluidly connected tothe outlet of the contact tank, a second inlet fluidly connected to asource of gas, a first floated solids outlet fluidly connected to thethird inlet of the aeration unit, a second floated solids outlet fluidlyconnected to the second inlet of the contact tank, and an effluentoutlet. The dissolved air flotation unit may be configured to treat thesecond mixed liquor with the gas to form the floated solids and theeffluent. The biological treatment unit may have a first inlet fluidlyconnected to the effluent outlet of the dissolved air flotation unit,and an outlet. The biological treatment unit may be configured to treatthe effluent from the dissolved air flotation unit to form a third mixedliquor.

In accordance with some embodiments, the system may further comprise afirst metering valve positioned upstream from the first floated solidsoutlet and the second floated solids outlet. In some embodiments, thesystem may further comprise a first controller operatively connected tothe first metering valve and configured to instruct the first meteringvalve to selectively dispense the floated solids to the aeration unitand the contact tank.

The contact tank may have a third inlet and the biological treatmentunit may have a second inlet. In some embodiments, the system mayfurther comprise a solids-liquid separation unit having an inlet fluidlyconnected to the outlet of the biological treatment unit, a solids-leaneffluent outlet, a first return activated sludge outlet fluidlyconnected to the third inlet of the contact tank, and a second returnactivated sludge outlet fluidly connected to the second inlet of thebiological treatment unit. The solids-liquid separation unit may beconfigured to treat solids from the third mixed liquor to form thesolids-lean effluent and the return activated sludge.

In accordance with certain embodiments, the solids-liquid separationunit may have a third return activated sludge outlet. In someembodiments, the aeration unit may have a fourth inlet fluidly connectedto the third return activated sludge outlet.

The system may further comprise a second metering valve positionedupstream from the first return activated sludge outlet and the thirdreturn activated sludge outlet. In some embodiments, the system mayfurther comprise a second controller operatively connected to the secondmetering valve and configured to instruct the second metering valve toselectively dispense the return activated sludge to the contact tank andthe aeration unit.

In some embodiments, the contact tank may have a fourth inlet fluidlyconnectable to the source of the wastewater to be treated.

The dissolved air flotation unit may have a third floated solids outlet.In some embodiments, the system may further comprise an anaerobicdigester having an inlet fluidly connected to the third floated solidsoutlet.

In accordance with certain embodiments, the system may comprise a firstsubsystem including the aeration unit, contact tank, and dissolved airflotation unit. The first subsystem may be physically separated from asecond subsystem including the biological treatment unit and thesolids-liquid separation unit. The aeration unit and the contact tankmay be included in a same treatment tank.

In accordance with another aspect, there is provided a method oftreating wastewater. The method may comprise directing the wastewaterinto an aeration unit and aerating the wastewater with oxygen to form anaerated mixed liquor. The method may comprise directing the aeratedmixed liquor into a contact tank and mixing the aerated mixed liquorwith an activated sludge to form an activated mixed liquor. The methodmay comprise directing the activated mixed liquor into a dissolved airflotation unit and separating the activated mixed liquor to form afloated biosolids and an effluent. The method may comprise selectivelydirecting a first portion of the floated biosolids into the aerationunit and a second portion of the floated biosolids into the contacttank. The method may comprise directing the effluent into a biologicaltreatment unit and biologically treating the effluent to form abiologically treated mixed liquor.

In some embodiments, the method may further comprise directing thebiologically treated mixed liquor into a solids-liquid separation unitand separating the biologically treated mixed liquor to form asolids-lean effluent and the activated sludge. The method may comprisedirecting a first portion of the activated sludge into the biologicaltreatment unit.

The method may further comprise selectively directing a second portionof the activated sludge into the contact tank and a third portion of theactivated sludge into the aeration unit.

In some embodiments, the method may further comprise directing a secondportion of the wastewater into the contact tank.

The method may comprise directing the wastewater having at least about5% solids content into the aeration unit. The method may comprisedirecting the wastewater having at least about 20% solids content intothe aeration unit.

In accordance with certain embodiments, a minority fraction ofbiological oxygen demand in the wastewater introduced into the aerationunit may be oxidized in the aeration unit. In some embodiments, agreater amount of biological oxygen demand in the wastewater introducedinto the aeration unit may be oxidized in the aeration unit thanbiological oxygen demand in the aerated mixed liquor is oxidized in thecontact tank.

In accordance with another aspect, there is provided a method ofretrofitting a wastewater treatment system. The wastewater treatmentsystem may comprise a contact tank, a dissolved air flotation unit, anda biological treatment unit. The method may comprise providing anaeration unit having a first inlet fluidly connectable to a source ofwastewater to be treated, a second inlet fluidly connectable to a sourceof oxygen, a third inlet, and an outlet. The aeration unit may beconfigured to aerate the wastewater with the oxygen to form an aeratedmixed liquor. The method may comprise fluidly connecting the outlet ofthe aeration unit to the contact tank. The method may comprise fluidlyconnecting the third inlet to a floated solids outlet of the dissolvedair flotation unit.

In some embodiments, the wastewater treatment system may furthercomprise a first metering valve positioned downstream from the floatedsolids outlet. The method may comprise installing a first controlleroperatively connected to the first metering valve and configured toinstruct the first metering valve to selectively dispense the floatedsolids to the aeration unit and the contact tank.

In some embodiments, the wastewater treatment system may furthercomprise a solids-liquid separation unit. The method may furthercomprise providing an aeration unit having a fourth inlet. The methodmay further comprise fluidly connecting the fourth inlet to a returnactivated sludge outlet of the solids-liquid separation unit.

The wastewater treatment system may further comprise a second meteringvalve positioned downstream from the return activated sludge outlet. Themethod may further comprise installing a second controller operativelyconnected to the second metering valve and configured to instruct thesecond metering valve to selectively dispense the return activatedsludge to the aeration unit and the contact tank.

In accordance with another aspect, there is provided a method offacilitating treatment of high solids content wastewater with awastewater treatment system. The wastewater treatment system maycomprise a contact tank, a dissolved air flotation unit, and abiological treatment unit. The method may comprise providing an aerationunit having a first inlet, a second inlet, a third inlet, and an outlet.The aeration unit may be configured to aerate the wastewater with oxygento form an aerated mixed liquor. The method may comprise fluidlyconnecting the first inlet to a source of the wastewater to be treated.The method may comprise fluidly connecting the second inlet to a sourcethe oxygen. The method may comprise fluidly connecting the outlet to aninlet of the contact tank. The method may comprise instructing a user tooperate the dissolved air flotation unit to generate the floatedbiosolids. The method may comprise instructing the user to direct atleast a first portion of the floated biosolids to the aeration unit.

In some embodiments, the method may further comprise instructing theuser to operate the wastewater treatment system to treat wastewaterhaving at least about 5% solids content. The method may further compriseinstructing the user to operate the wastewater treatment system to treatwastewater having at least about 20% solids content.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a block flow diagram of a wastewater treatment system inaccordance with an embodiment;

FIG. 2 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 3 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 4 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 5 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 6 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 7 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 8 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 9 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 10 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 11 illustrates a first set of results of a test of a system inaccordance with an embodiment;

FIG. 12 illustrates a second set of results of a test of a system inaccordance with an embodiment;

FIG. 13 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment;

FIG. 14 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment; and

FIG. 15 is a block flow diagram of a wastewater treatment system inaccordance with another embodiment.

DETAILED DESCRIPTION

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” “having,” “containing,”“involving,” and variations thereof herein is meant to encompass theitems listed thereafter and equivalents thereof as well as additionalitems.

As the term is used herein, an “upstream” unit operation refers to afirst unit operation which is performed upon a fluid undergoingtreatment prior to a second unit operation. Similarly, an “upstream”treatment vessel or portion thereof refers to a first treatment vesselor portion thereof in which a first unit operation is performed prior toa second unit operation performed in a second treatment vessel orportion thereof. A “downstream” unit operation refers to a second unitoperation which is performed upon a fluid undergoing treatmentsubsequent to a first unit operation. Similarly, a “downstream”treatment vessel or portion thereof refers to a second treatment vesselor portion thereof in which a second unit operation is performedsubsequent to a first unit operation performed in a first treatmentvessel or portion thereof. An upstream unit operation and/or treatmentvessel having an outlet in “direct fluid communication” with an inlet ofa downstream unit operation and/or treatment vessel directs materialoutput from the outlet of the upstream unit operation and/or treatmentvessel into the inlet of the downstream unit operation and/or treatmentvessel without any intervening operations performed on the material. Afirst unit operation and/or treatment vessel described herein as beingin fluid communication with a second unit operation and/or treatmentvessel should be understood as being in direct fluid communication withthe second unit operation and/or treatment vessel unless explicitlydescribed as otherwise. Conduits which provide fluid communicationbetween a first and a second unit operation and/or treatment vessel areto be understood as providing direct fluid communication between thefirst and second unit operation and/or treatment vessel unlessexplicitly described as otherwise.

Various unit operations and/or treatment vessels disclosed hereinseparate fluid and/or sludge into a solids-rich portion and asolids-lean portion wherein the solid-lean portion has a lowerconcentration of solids than the solids-rich portion. As the term isused herein, an “effluent” of a unit operation and/or treatment vesselrefers to the solids-lean portion of the separated fluid and/or sludge.“Recycle” of material refers to directing material from an outlet of adownstream unit operation and/or treatment vessel to an inlet of a unitoperation and/or treatment vessel upstream of the downstream unitoperation and/or treatment vessel.

U.S. Pat. Nos. 8,808,544 and 10,131,550, titled “ContactStabilization/Prime Float Hybrid” and “Enhanced Biosorption ofWastewater Organics Using Dissolved Air Flotation with Solids Recycle,”respectively, are incorporated herein by reference in their entiretiesfor all purposes.

Aspects and embodiments of the present invention are directed towardsystems and methods for treating wastewater. As used herein the term“wastewater” includes, for example, municipal wastewater, industrialwastewater, agricultural wastewater, and any other form of liquid to betreated containing undesired contaminants. Aspects and embodiments ofthe present invention may be utilized for primary wastewater treatment,secondary wastewater treatment, or both. Aspects and embodiments of thepresent invention may remove sufficient contaminants from wastewater toproduce product water that may be used for, for example, irrigationwater, potable water, cooling water, boiler tank water, or for otherpurposes.

In some embodiments, the apparatus and methods disclosed herein provideadvantages with regard to, for example, capital costs, operationalcosts, and environmental-friendliness as compared to conventionalbiological wastewater treatment systems. In some embodiments a dissolvedair flotation system is included in a main stream of wastewater enteringa biological wastewater treatment system. The dissolved air floatationsystem may remove a significant amount of biological oxygen demand, forexample, particulate biological oxygen demand, from wastewater prior tothe wastewater entering the biological treatment portion of thewastewater treatment system. This provides for a reduction in the sizeof the biological treatment portion of the wastewater treatment systemfor a given wastewater stream as compared to a conventional wastewatertreatment system and a commensurate reduced capital cost for the overallsystem. Utilization of the dissolved air flotation system also reducesthe requirement for aeration in the biological treatment portion of thetreatment system to effect oxidation of the biological oxygen demand ofthe wastewater, reducing operating costs. The amount of waste sludgegenerated by the biological treatment portion of the treatment system isalso reduced, reducing the amount of waste which would need to bedisposed of or otherwise further treated. The material removed from thewastewater in the dissolved air flotation system may be utilized toproduce energy, for example, in the form of biogas in a downstreamanaerobic digestion system. The biogas may be used to provide salableenergy through combustion or through use in, for example, fuel cells.

In accordance with an embodiment of the present invention there isprovided a method of facilitating increased operating efficiency of awastewater treatment system. The method comprises configuring adissolved air flotation (DAF) unit in a wastewater treatment system influid communication between a contact tank and a biological treatmentunit to remove solids from a portion of a first mixed liquor output fromthe contact tank prior to the portion of the first mixed liquor enteringthe biological treatment unit and to recycle at least a portion of thesolids to the contact tank, the recycle of the at least a portion of thesolids to the contact tank reducing an amount of biological oxygendemand to be treated in the biological treatment unit as compared to thewastewater treatment system operating in the absence of recycling the atleast a portion of the solids to the contact tank.

In some embodiments, greater than 50% of the solids are recycled fromthe DAF unit to the contact tank.

In some embodiments, the method comprises recycling solids from the DAFunit to the contact tank in an amount sufficient to increase biogasproduction of an anaerobic digester of the wastewater treatment systemhaving an inlet in fluid communication with an outlet of the DAF unit,at least a second portion of the solids removed in the DAF unit beingdirected into the anaerobic digester.

In some embodiments, the method comprises recycling solids from the DAFunit to the contact tank in an amount sufficient to reduce the energyconsumption of the wastewater treatment system.

In accordance with an embodiment of the present invention there isprovided a wastewater treatment system. The wastewater treatment systemcomprises a contact tank having a first inlet, a second inlet, and anoutlet and a dissolved air flotation tank having an inlet in fluidcommunication with the outlet of the contact tank, a first outlet, and asecond outlet. The wastewater treatment system further comprises anaerated anoxic tank having a first inlet in fluid communication with theoutlet of the contact tank, a second inlet, and an outlet and aerobictank having a first inlet in fluid communication with the outlet of theaerated anoxic tank, a second inlet in fluid communication with thefirst outlet of the dissolved air flotation tank, and an outlet. Thewastewater treatment system further comprises a clarifier having aninlet in fluid communication with the outlet of the aerobic tank and anoutlet in fluid communication with the second inlet of the contact tankand with the second inlet of the aerated anoxic tank.

In accordance with another embodiment of the present invention there isprovided a method of treating wastewater. The method comprisesintroducing the wastewater into a contact tank, mixing the wastewaterwith activated sludge in the contact tank to form a mixed liquor,transporting a first portion of the mixed liquor to a dissolved airflotation tank, separating the first portion of the mixed liquor in thedissolved air flotation tank to form a dissolved air flotation tankeffluent and waste biosolids, transporting a second portion of the mixedliquor to an aerated anoxic treatment tank, biologically treating thesecond portion of the mixed liquor in the aerated anoxic treatment tankto form an anoxic mixed liquor, transporting the anoxic mixed liquor toan aerobic treatment tank, transporting the dissolved air flotation tankeffluent to the aerobic treatment tank, biologically treating the anoxicmixed liquor and the dissolved air flotation tank effluent in theaerobic treatment tank to form an aerobic mixed liquor, transporting theaerobic mixed liquor to a clarifier, separating the aerobic mixed liquorin the clarifier to form a clarified effluent and a return activatedsludge, recycling a first portion of the return activated sludge to thecontact tank, and recycling a second portion of the return activatedsludge to the aerated anoxic treatment tank.

In accordance with an embodiment of the present invention there isprovided a wastewater treatment system. The wastewater treatment systemcomprises a contact tank having a first inlet configured to receivewastewater to be treated, a second inlet, and an outlet. The contacttank is configured to mix the wastewater to be treated with activatedsludge to form a first mixed liquor. The system further comprises a DAFunit having an inlet in fluid communication with the outlet of thecontact tank, a solids outlet, a DAF unit effluent outlet, and a gasinlet. The gas inlet is configured to introduce gas into the DAF unit tofacilitate the flotation of suspended matter from the first mixed liquorand the removal of the suspended matter from the DAF unit. The solidsoutlet is in fluid communication with the first inlet of the contacttank and configured to transfer at least a portion of the suspendedmatter from the DAF unit to the first inlet of the contact tank. Thesystem further comprises a biological treatment unit having a firstinlet in fluid communication with the outlet of the contact tank, asecond inlet, a third inlet in fluid communication with the DAF uniteffluent outlet, and an outlet. The biological treatment unit isconfigured to biologically break down organic components of the firstmixed liquor and of an effluent from the DAF unit to form a second mixedliquor. The system further comprises a clarifier having an inlet influid communication with the outlet of the biological treatment unit, aneffluent outlet, and a return activated sludge outlet in fluidcommunication with the second inlet of the contact tank and with thesecond inlet of the biological treatment unit. The clarifier isconfigured to output a clarified effluent through the effluent outletand a return activated sludge though the return activated sludge outlet.

In accordance with some aspects of the wastewater treatment system, thebiological treatment unit includes an aerated anoxic region having afirst inlet in fluid communication with the outlet of the contact tank,a second inlet, and an outlet and an aerobic region having a first inletin fluid communication with the outlet of the aerated anoxic region, asecond inlet in fluid communication with the DAF unit effluent outlet,and an outlet.

In accordance with some aspects of the wastewater treatment system, theaerated anoxic region and the aerobic region are included in a sametreatment tank.

In accordance with some aspects of the wastewater treatment system, theaerated anoxic region and the aerobic region are separated by apartition.

In accordance with some aspects of the wastewater treatment system, theaerated anoxic region is included in a first treatment tank and theaerobic region is included in a second treatment tank distinct from thefirst treatment tank.

In accordance with some aspects of the wastewater treatment system, thewastewater treatment system comprises a first sub-system including thecontact tank and the DAF unit which is physically separated from asecond sub-system including the biological treatment unit and theclarifier.

In accordance with some aspects of the wastewater treatment system, thecontact tank and the aerated anoxic region are included in a same tank.

In accordance with some aspects of the wastewater treatment system, thewastewater treatment system further comprises an anaerobic digesterhaving an inlet in fluid communication with the solids outlet of the DAFunit and an outlet.

In accordance with some aspects of the wastewater treatment system, theoutlet of the anaerobic digester is in fluid communication with at leastone of the contact tank and the biological treatment unit.

In accordance with some aspects of the wastewater treatment system, thewastewater treatment system further comprises a primary clarifier havingan inlet in fluid communication with a source of the wastewater to betreated and a solids-lean outlet in fluid communication with the contacttank.

In accordance with some aspects of the wastewater treatment system, thewastewater treatment system further comprises a thickener having aninlet in fluid communication with a solids-rich outlet of the primaryclarifier and an outlet in fluid communication with the anaerobicdigester.

In accordance with some aspects of the wastewater treatment system, theprimary clarifier further comprises a solids-rich outlet in fluidcommunication with the DAF unit.

In accordance with another embodiment of the present invention there isprovided a method of treating wastewater. The method comprisesintroducing the wastewater into a contact tank including an activatedsludge, mixing the wastewater with activated sludge in the contact tankto form a mixed liquor, and directing a first portion of the mixedliquor to a DAF unit. The method further comprises separating the firstportion of the mixed liquor in the DAF unit to form a DAF unit effluentand separated biosolids, directing at least a portion of the separatedbiosolids from the DAF unit to the contact tank, directing a secondportion of the mixed liquor to a biological treatment unit, directingthe DAF unit effluent to the biological treatment unit, biologicallytreating the mixed liquor and the DAF unit effluent in the biologicaltreatment unit to form a biologically treated mixed liquor, anddirecting the biologically treated mixed liquor to a clarifier. Themethod further comprises separating the biologically treated mixedliquor in the clarifier to form a clarified effluent and a returnactivated sludge, recycling a first portion of the return activatedsludge to the contact tank, recycling a second portion of the returnactivated sludge to the biological treatment unit, and directing theclarified effluent to a treated wastewater outlet.

In accordance with some aspects of the method of treating wastewaterwherein the biological treatment unit includes an aerated anoxictreatment unit and an aerobic treatment unit, the method furthercomprises directing the second portion of the mixed liquor to theaerated anoxic treatment unit, treating the second portion of the mixedliquor in the aerated anoxic treatment unit to form an anoxic mixedliquor, directing the anoxic mixed liquor to the aerobic treatment unit,directing the DAF unit effluent to the aerobic treatment unit, treatingthe anoxic mixed liquor and the DAF unit effluent in the aerobictreatment tank to form an aerobic mixed liquor, directing the aerobicmixed liquor to the clarifier, separating the aerobic mixed liquor inthe clarifier to form the clarified effluent and the return activatedsludge, and recycling the second portion of the return activated sludgeto the aerated anoxic treatment unit.

In accordance with some aspects of the method of treating wastewater,the first portion of the return activated sludge and the second portionof the return activated sludge comprise about 100% of all returnactivated sludge formed in the clarifier.

In accordance with some aspects of the method of treating wastewater,the first portion of the return activated sludge comprises between about10% and about 20% of all return activated sludge recycled from theclarifier.

In accordance with some aspects of the method of treating wastewater,the first portion of the mixed liquor comprises between about one thirdand about two thirds of all mixed liquor formed in the contact tank.

In accordance with some aspects of the method of treating wastewater,the DAF unit removes between about 60% and about 100% of suspendedsolids in the first portion of the mixed liquor from the first portionof the mixed liquor.

In accordance with some aspects of the method of treating wastewater, anamount of suspended solids removed in the DAF unit is adjusted basedupon a concentration of a bacteria in the biological treatment unit.

In accordance with some aspects of the method of treating wastewater,the DAF unit removes between about 40% and about 80% of biologicaloxygen demand in the first portion of the mixed liquor from the firstportion of the mixed liquor.

In accordance with some aspects of the method of treating wastewater,the method further comprises treating at least a portion of the wastebiosolids in an anaerobic digester to produce an anaerobically digestedsludge.

In accordance with some aspects of the method of treating wastewater,the method further comprises recycling at least a portion of theanaerobically digested sludge to at least one of the contact tank andthe biological treatment unit.

In accordance with some aspects of the method of treating wastewater,the method further comprises separating the water to be treated into asolids-lean portion and a solids-rich portion, directing the solids-richportion into a thickener to produce a solids-rich output and asolids-lean effluent, directing the solids-lean portion into the contacttank, directing the solids-rich output from the thickener into theanaerobic digester, and directing the solids-lean effluent of thethickener into the contact tank.

In accordance with another embodiment of the present invention there isprovided method of facilitating increased operating efficiency of awastewater treatment system. The method comprises providing a DAF unitin a wastewater treatment system in fluid communication between acontact tank and a biological treatment unit, the DAF unit configured toremove solids from a portion of a first mixed liquor output from thecontact tank prior to the portion of the first mixed liquor entering thebiological treatment unit and to recycle at least a portion of thesolids to the contact tank, reducing the amount of biological oxygendemand to be treated in the biological treatment unit as compared to thewastewater treatment system operating in the absence of the DAF unit,and providing for a solids-liquid separation unit in fluid communicationdownstream of the biological treatment unit to recycle a returnactivated sludge formed from a mixed liquor output from the biologicaltreatment unit to the contact tank.

In accordance with some aspects, the method further comprises providingfor between about 10% and about 20% of the return activated sludgeformed to be recycled to the contact tank.

In accordance with some aspects, the method further comprises adjustingan amount of return activated sludge recycled to the contact tank basedupon a concentration of a bacteria in the biological treatment unit.

In accordance with some aspects, the method further comprises providingan anaerobic digester having an inlet in fluid communication with anoutlet of the DAF unit and an outlet in fluid communication with atleast one of an inlet of the contact tank and an inlet of the biologicaltreatment unit.

A first embodiment, indicated generally at 100, is illustrated inFIG. 1. Wastewater from a source of wastewater 105 is directed into acontact tank 110 through an inlet of the contact tank. In the contacttank 110, the wastewater is mixed with activated sludge recycled througha conduit 175 from a downstream biological treatment process describedbelow. In some embodiments, the contact tank 110 is aerated tofacilitate mixing of the wastewater and the activated sludge. Theaeration gas may be an oxygen containing gas, for example, air. Thecontact tank 110 may be provided with sufficient oxygen such thataerobic conditions are maintained in at least a portion of the contacttank 110. For example, the contact tank 110 may be aerated. In otherembodiments, the contact tank 110 may not be aerated. Suspended anddissolved solids in the wastewater, including oxidizable biologicalmaterials (referred to herein as Biological Oxygen Demand, or BOD), areadsorbed/absorbed into the activated sludge in the contact tank, forminga first mixed liquor. A portion of the BOD may also be oxidized in thecontact tank 110. The residence time of the wastewater in the contacttank may be sufficient for the majority of the BOD to beadsorbed/absorbed by the activated sludge, but no so long as for asignificant amount of oxidation of the BOD to occur. In someembodiments, for example, less than about 10% of the BOD entering thecontact tank 110 is oxidized in the contact tank. The residence time ofthe wastewater in the contact tank is in some embodiments from about 30minutes to about two hours, and in some embodiments, from about 45minutes to about one hour. The residence time may be adjusted dependingupon factors such as the BOD of the influent wastewater. A wastewaterwith a higher BOD may require longer treatment in the contact tank 110than wastewater with a lower BOD.

A first portion of the first mixed liquor formed in the contact tank isdirected into a dissolved air flotation (DAF) system 120 through conduit114. The DAF system may include a vessel, tank, or other open or closedcontainment unit configured to perform a dissolved air flotationoperation as described below. For the sake of simplicity a dissolved airflotation system will be referred to herein as a “DAF unit.” The DAFunit 120 may function as both a thickener and a clarifier. FIG. 1illustrates two DAF units 120 operating in parallel, however, otherembodiments may have a single DAF unit or more than two DAF units.Providing multiple DAF units provides for the system to continueoperation if one of the DAF units is taken out of service for cleaningor maintenance.

Before entering the DAF unit(s), air or another gas may be dissolved inthe first mixed liquor under pressure. The pressure may be released asthe first mixed liquor enters the DAF unit(s) 120, resulting in the gascoming out of solution and creating bubbles in the mixed liquor. In someembodiments, instead of dissolving gas into the first mixed liquor, afluid, for example, water having a gas, for example, air, dissolvedtherein, is introduced into the DAF unit(s) 120 with the first mixedliquor. Upon the mixing of the first mixed liquor and the gas-containingfluid, bubbles are produced. The bubbles formed in the DAF unit(s) 120adhere to suspended matter in the first mixed liquor, causing thesuspended matter to float to the surface of the liquid in the DAFunit(s) 120, where it may be removed by, for example, a skimmer.

In some embodiments, the first mixed liquor is dosed with a coagulant,for example, ferric chloride or aluminum sulfate prior to or afterintroduction into the DAF unit(s) 120. The coagulant facilitatesflocculation of suspended matter in the first mixed liquor.

In the DAF unit(s) 120 at least a portion of the solids present in theinfluent first mixed liquor, including solids from the influentwastewater and from the recycled activated sludge, are removed by adissolved air flotation process. At least a portion of any oil that maybe present in the first mixed liquor may also be removed in the DAFunit(s) 120. In some embodiments, a majority, for example, about 60% ormore, about 75% or more, or about 90% or more of the suspended solids inthe first mixed liquor introduced into the DAF unit(s) 120 is removedand about 40% or more, for example, about 50% or more or about 75% ormore of the BOD is removed. Removal of the BOD may include enmeshmentand adsorption in the first mixed liquor and/or oxidation of the BOD andthe formation of reaction products such as carbon dioxide and water. Inother embodiments, up to about 100% of the suspended solids is removedin the DAF unit(s) 120 and a majority, for example, up to about 80% ofthe BOD is removed.

In some embodiments, suspended solids removed in the DAF unit(s) 120 aresent out of the system as waste solids through a conduit 125. Thesewaste solids may be disposed of, or in some embodiments, may be treatedin a downstream process, for example, an anaerobic digestion process oranaerobic membrane bioreactor to produce useful products, for example,biogas and/or usable product water.

In other embodiments, at least a portion of the suspended solids removedin the DAF unit(s) 120 are recycled back to the contact tank 110 throughconduits 125 and 126. Conduit 126 may branch off of conduit 125 asillustrated, or may be connected to a third outlet of the DAF unit(s)120, in which case suspended solids removed in the DAF unit(s) 120 arerecycled back to the contact tank 110 through conduit 126 only. Theamount of solids recycled from DAF unit(s) 120 to the contact tank 110may range from about 1% to about 100% of a total amount of solidsremoved from the first mixed liquor in the DAF unit(s) 120. The amountof solids recycled from DAF unit(s) 120 to the contact tank 110 may be amajority of a total amount of solids removed from the first mixed liquorin the DAF unit(s) 120, for example, greater than about 50%, betweenabout 50% and about 95%, or between about 60% and about 80% of the totalamount of solids removed from the first mixed liquor in the DAF unit(s)120.

Recycling solids removed in the DAF unit(s) 120 to the contact tank 110is counter to the conventional operation of wastewater treatment systemsincluding DAF units. Typically, DAF units are utilized in wastewatertreatment systems to remove solids from the wastewater, thus reducingthe need for biological treatment of these removed solids and reducingthe energy requirements of the wastewater treatment system by, forexample, reducing the amount of air needed to be supplied to an aeratedbiological treatment vessel to oxidize the removed solids. It is counterto conventional operation of wastewater treatment systems tore-introduce floated solids separated from mixed liquor from a contacttank in DAF unit(s) back to the contact tank. Typically, after solidsare separated from mixed liquor from a contact tank in DAF unit(s),reintroducing the separated solids into mixed liquor in the contact tankand forcing the solids to go through the same separation process in theDAF unit(s) would reduce the efficiency of the system. Such a solidsrecycle from DAF unit(s) to a contact tank directly upstream of the DAFunit(s) would cause a need for a greater amount of contact tank capacityand a greater amount of DAF unit capacity. Such a solids recycle fromDAF unit(s) to a contact tank directly upstream of the DAF unit(s) wouldalso require more air flow to the DAF unit(s) to remove the recycledsolids from the mixed liquor in addition to any solids that would bepresent in the absence of the solids recycle. It has been discovered,however, that benefits may be achieved by the counterintuitivere-introduction of solids removed in DAF unit(s) back into the contacttank of a wastewater treatment system from which mixed liquor issupplied to the DAF unit(s).

For example, by recycling the solids removed by the DAF unit(s) 120 tothe contact tank 110, the amount of total suspended solids (TSS) in thecontact tank 110 may be increased as compared to methods not including arecycle of solids from the DAF unit(s) 120 to the contact tank 110. Theincreased TSS level in the contact tank 110 may provide for additionalsoluble BOD to be adsorbed in the contact tank 110 as compared to acontact tank 110 having a lower level of TSS. In some embodiments, adesirable TSS level in the contact tank 110 may be between about 1,200mg/L and about 3,500 mg/L.

The removal of the additional soluble BOD in the contact tank 110 due tothe higher TSS level in the contact tank 110, resulting from the recycleof solids from the DAF unit(s) 120 to the contact tank 110, provides forthe removal of this additional BOD as solids in the DAF unit(s) 120. Theadditional BOD removed as solids in the DAF unit(s) 120 may be directedto an anaerobic digester (for example, anaerobic digester 490illustrated in FIG. 4) rather than an aerated biological treatment unit(for example, biological treatment unit 130), thus reducing the need foraeration power in the biological treatment unit and increasing theamount of biogas that could be produced in the anaerobic digester.

When supplied with recycled solids from the DAF unit(s) 120, the contacttank 110 may have a hydraulic retention time (HRT) of between about 15minutes and about one hour and a solids retention time (SRT) of betweenabout 0.5 days and about two days to effectively adsorb soluble BOD. Inother embodiments, the SRT in the contact tank may be between about 0.2and about 0.4 days. When the contact tank 110 includes TSS in a range ofbetween about 1,200 mg/L and about 3,500 mg/L, a sludge age (SRT) in thecontact tank may range from about one to about two days.

Recycling solids removed in the DAF unit(s) 120 to the contact tank 110provides for the contact tank 110 to function as a high rate activatedsludge system while the DAF unit(s) 120 function a solids-liquidseparator. Recycling solids removed in the DAF unit(s) 120 to thecontact tank 110 provides for greater oxidation of BOD in the contacttank 110 than in systems where solids removed from the DAF unit(s) 120are not recycled to the contact tank because the solids recycled to thecontact tank includes living bacteria capable of oxidizing BOD. Forexample, in systems and methods where solids removed in the DAF unit(s)120 are recycled to the contact tank 110, oxidation of greater thanabout 10% of the BOD in wastewater influent to the contact tank 110 maybe oxidized in the contact tank 110. Recycling solids removed in the DAFunit(s) 120 to the contact tank 110 may thus reduce the amount of BODthat needs to be treated in downstream unit operations, for example, inthe biological treatment unit 130 discussed below, thus reducing thepower requirements for the downstream unit operations. The SRT of thecontact tank 110 may be adjusted to optimize BOD removal of particulate,colloidal, and soluble BOD fractions.

Effluent from the DAF unit(s) 120 is directed through conduit 124 intothe biological treatment unit 130, which may include one or moretreatment tanks. In some embodiments, the biological treatment unit 130may comprise a contact stabilization vessel. A portion of the effluentmay be recycled (recycle system not shown in FIG. 1) to supply gasbubbles to the DAF unit(s) 120. A gas may be dissolved into the recycledportion of effluent, which is then directed back into the DAF unit(s)120 and mixed with influent first mixed liquor.

A second portion of the first mixed liquor formed in the contact tank isdirected into the biological treatment unit 130 through a conduit 115.In some embodiments, about a half of the first mixed liquor formed inthe contact tank is directed into the DAF unit(s) 120 and about a halfof the first mixed liquor formed in the contact tank is directed throughthe conduit 115 into the biological treatment unit 130. In otherembodiments, between about one third and two thirds of the first mixedliquor formed in the contact tank is directed into the DAF unit(s) 120and the remainder of the first mixed liquor formed in the contact tankis directed through the conduit 115 into the biological treatment unit130. The amount of the first mixed liquor directed into the DAF unit(s)120 as opposed to the biological treatment unit 130 may be varied basedupon such factors as the concentration of the first mixed liquor and theeffectiveness of the first mixed liquor at enmeshing BOD in the contacttank 110.

For example, if it was desired to remove a greater rather than a lesseramount of solids in the DAF unit(s) 120, a greater fraction of the firstmixed liquor from the contact tank would be directed to the DAF unit(s)120 when the first mixed liquor had a lower rather than a higherconcentration of solids. Similarly, if it was desired to remove agreater rather than a lesser amount of BOD in the DAF unit(s) 120, agreater fraction of the first mixed liquor from the contact tank wouldbe directed to the DAF unit(s) 120 when the first mixed liquor had alesser rather than a greater effectiveness at enmeshing BOD in thecontact tank.

In the biological treatment unit 130, the effluent from the DAF unit(s)120 and the first mixed liquor formed in the contact tank 110 arecombined to form a second mixed liquor which is biologically treated. Insome embodiments, biological treatment of the second mixed liquor in thebiological treatment unit 130 includes oxidation of BOD in the secondmixed liquor. To this end, oxygen may be supplied to the second mixedliquor in the biological treatment unit 130 by aeration with an oxygencontaining gas, for example, air. In some embodiments, the biologicaltreatment unit 130 is supplied with sufficient oxygen for aerobicconditions to be created in the biological treatment unit 130. In otherembodiments, the amount of oxygen supplied is insufficient to meet theentire oxygen demand of the second mixed liquor, and the biologicaltreatment unit 130, or at least a portion thereof, may be maintained inan anoxic or anaerobic condition. Nitrification and denitrification ofthe second mixed liquor may occur in different portions of the aeratedbiological treatment unit 130. The residence time of the second mixedliquor in the biological treatment unit 130 may be sufficient to oxidizesubstantially all BOD in the second mixed liquor. Residence time for thesecond mixed liquid in the biological treatment unit 130 may be fromabout three to about eight hours. This residence time may be increasedif the influent wastewater to be treated and/or the second mixed liquorcontains a high level of BOD or decreased if the influent wastewater tobe treated and/or the second mixed liquor includes a low level of BOD.

Biologically treated mixed liquor from the biological treatment unit 130is directed through a conduit 135 into a separation apparatus, which mayinclude, for example, a clarifier 140, a gravity separation apparatus,and/or another form of separation apparatus. Effluent from the clarifier140 may be directed to a product water outlet through a conduit 145 orbe sent on for further treatment. Activated sludge separated fromeffluent in the clarifier may be recycled back upstream to a wastewaterinlet of the system, the source of wastewater, the contact tank 110through conduits 155 and 175, and/or the biological treatment unit 130through conduits 155 and 165. In some embodiments 100% of the activatedsludge separated in the clarifier is recycled upstream. In someembodiments between about 10% and about 20% of the recycled sludge isdirected to the wastewater inlet and contact tank through the conduit175 and between about 80% and 90% of the recycled sludge is directedinto the biological treatment unit 130 through the conduit 165. Theamount of recycled sludge directed to the wastewater inlet and contacttank through the conduit 175 may be set at a higher end of this rangewhen the incoming wastewater has a high level of BOD and/or when therecycled sludge is less rather than more effective at enmeshing BOD inthe contact tank 110. The amount of recycled sludge directed to thewastewater inlet and contact tank through the conduit 175 may be set ata lower end of this range when the incoming wastewater has a low levelof BOD and/or when the recycled sludge is more rather than lesseffective at enmeshing BOD in the contact tank 110.

The amount of activated sludge separated in the clarifier 140 which isrecycled to the contact tank 110 and/or biological treatment unit 130may also be adjusted based on a fraction of the first mixed liquor fromthe contact tank 110 which is directed to the DAF unit(s) 120, theamount of activated sludge which is removed in the DAF units(s) 120,and/or the amount of activated sludge removed in the DAF units(s) 120which is recycled to the contact tank 110. The amount of activatedsludge which is recycled to the contact tank 110 and/or biologicaltreatment unit 130 may be an amount equal to or greater than an amountrequired to maintain a desired population of bacteria in the biologicaltreatment unit 130 to perform biological treatment of the second mixedliquor within a desired timeframe and/or to protect against depletion ofthe bacterial population in the event of temporary disruptions in theoperation of the treatment system. For example, the amounts of activatedsludge which is recycled to the contact tank 110 or biological treatmentunit 130 may be set such that sufficient bacteria containing solids arepresent in the biological treatment unit 130 to result in a SRT ofbetween about one and about 10 days in the biological treatment unit130. Similarly, an amount or fraction of the first mixed liquor directedinto the DAF unit(s) 120 may be adjusted based on the amount ofactivated sludge recycled from the clarifier 140, the efficiency ofremoval of solids in the DAF unit(s) 120 and/or the concentration of oneor more types of bacteria in the biological treatment unit 130 to, forexample, establish or maintain a desired population of bacteria in thebiological treatment unit 130.

In the embodiment illustrated in FIG. 1, and in the additionalembodiments described below, it should be understood that the variousconduits illustrated may be provided with, for example, pumps, valves,sensors, and control systems as needed to control the flow of fluidstherethrough. These control elements are not illustrated in the figuresfor the sake of simplicity.

In another embodiment, indicated generally at 200 in FIG. 2, thebiological treatment unit 130 includes an aerobic region 150 and anaerated anoxic region 160. The aerobic region 150 is in fluidcommunication downstream of the aerated anoxic region 160 and receivesbiologically treated anoxic mixed liquor from the aerated anoxic region.In some embodiments, the aerobic region 150 may be formed in a samevessel or tank as the aerated anoxic region 160 and separated therefromby a partition or weir 195. In other embodiments, the aerobic region 150may be physically separate from the aerated anoxic region 160. Forexample, the aerobic region 150 and the aerated anoxic region 160 mayoccupy distinct vessels or tanks or may be otherwise separated from oneanother. In further embodiments the contact tank 110 may be combinedwith the aerated anoxic region 160 in the same tank.

In the system of FIG. 2 effluent from the DAF unit(s) 120 is directedinto the aerobic region 150 without first passing through the aeratedanoxic region 160. In other embodiments, the effluent from the DAFunit(s) 120 may be introduced into the aerated anoxic region 160 andthen directed into the aerobic region 150.

Another embodiment, indicated generally at 300, is illustrated in FIG.3. In this embodiment, the wastewater treatment system 300 is brokeninto two separate but interconnected subsystems, one subsystem 300Aincluding a contact tank 210 and DAF unit(s) 220, and a second subsystem300B including a biological treatment unit 230 and a separationapparatus 240. In the first subsystem 300A influent wastewater from asource of wastewater 205A is directed into the contact tank 210. In thecontact tank, the wastewater is mixed with activated sludge recycledthrough a conduit 275 from a biological treatment process included insubsystem 300B described below. In some embodiments, the contact tank210 is aerated to facilitate mixing of the wastewater and the activatedsludge. Suspended and dissolved solids in the wastewater areadsorbed/absorbed into the activated sludge in the contact tank 210,forming a first mixed liquor. A portion of the BOD in the influentwastewater may be oxidized in the contact tank 210. The residence timeof the wastewater in the contact tank may be sufficient for the majorityof the BOD to be adsorbed/absorbed by the activated sludge, but no solong as for a significant amount of oxidation of the BOD to occur. Insome embodiments, for example, less than about 10% of the BOD enteringthe contact tank 210 is oxidized in the contact tank. The residence timeof the wastewater in the contact tank is in some embodiments from about30 minutes to about two hours, and in some embodiments, from about 45minutes to about one hour. The residence time may be adjusted dependingupon factors such as the BOD of the influent wastewater. A wastewaterwith a higher BOD may require longer treatment in the contact tank 210than wastewater with a lower BOD.

A first portion of the first mixed liquor formed in the contact tank isdirected into a DAF unit 220 through conduit 214. FIG. 3 illustrated twoDAF units 220 operating in parallel, however other embodiments may havea single DAF unit or more than two DAF units. Providing multiple DAFunits provides for the system to continue operation if one of the DAFunits is taken out of service for cleaning or maintenance. A secondportion of the first mixed liquor formed in the contact tank is directedinto the biological treatment unit 230 in the second subsystem 300Bthrough a conduit 215. In some embodiments, about a half of the firstmixed liquor formed in the contact tank is directed into the DAF unit(s)220 and about a half of the first mixed liquor formed in the contacttank is directed through the conduit 215 into the biological treatmentunit 230. In other embodiments, between about one third and two thirdsof the first mixed liquor formed in the contact tank is directed intothe DAF unit(s) 220 and the remainder of the first mixed liquor formedin the contact tank is directed through the conduit 215 into thebiological treatment unit 230. The amount of the first mixed liquordirected into the DAF unit(s) 220 as opposed to the biological treatmentunit 230 may be varied based upon such factors as the concentration ofthe first mixed liquor and the effectiveness of the first mixed liquorat enmeshing BOD in the contact tank 210.

In the DAF unit(s) 220 at least a portion of the solids present in theinfluent first mixed liquor, including solids from the influentwastewater and from the recycled activated sludge, are removed by adissolved air flotation process such as that described above withreference to DAF unit(s) 120. The removed suspended solids may be sentout of the system as waste solids through a waste conduit 225. Thesewaste solids may be disposed of or treated in a downstream process, forexample, an anaerobic digestion process or anaerobic membrane bioreactorto produce biogas and/or usable product water. Effluent from the DAFunit(s) 220 is directed to an outlet 224 from which it may be used asproduct water or sent on for further treatment.

In some embodiments, a portion of the suspended solids removed from thefirst mixed liquor in the DAF unit(s) 220 may be recycled to the contacttank 210 through conduits 225 and 226 in a similar manner as the recycleof suspended solids removed in the DAF unit(s) 120 to the contact tank110 described above with reference to FIG. 1.

In the second subsystem 300B, influent wastewater from a source ofwastewater 205B is introduced into the biological treatment unit 230.The source of wastewater 205B may be the same as or different from thesource of wastewater 205A. In the biological treatment unit 230 thewastewater and the first mixed liquor formed in the contact tank 210 arecombined to form a second mixed liquor which is biologically treated. Insome embodiments, biological treatment of the second mixed liquor in thebiological treatment unit 230 may include oxidation of BOD in the secondmixed liquor. To this end, oxygen may be supplied to the second mixedliquor in the biological treatment unit 230 by aeration with an oxygencontaining gas, for example, air. In some embodiments, the biologicaltreatment unit 230 is supplied with sufficient oxygen for aerobicconditions to be created in the biological treatment unit 230. In otherembodiments, the amount of oxygen supplied is insufficient to meet theentire oxygen demand of the second mixed liquor and the biologicaltreatment unit 230, or at least a portion thereof, may be maintained inan anoxic or anaerobic condition. Nitrification and denitrification ofthe second mixed liquor may occur in different portions of the aeratedbiological treatment unit 230.

Residence time for the second mixed liquid in the biological treatmenttank 230 may be from about three to about eight hours. This residencetime may be increased if the influent wastewater to be treated and/orthe second mixed liquor contains a high level of BOD or decreased if thewastewater and/or the second mixed liquor includes a low level of BOD.

Biologically treated mixed liquor from the biological treatment unit 230is directed through a conduit 235 into a separation apparatus, which mayinclude, for example, a clarifier 240. Effluent from the clarifier 240may be directed to a product water outlet through a conduit 245 or besent on for further treatment. Activated sludge separated from effluentin the clarifier may be recycled back upstream to the biologicaltreatment unit 230 and/or to the contact tank 210 in subsystem 300Athrough a conduit 255. In some embodiments about 100% of the activatedsludge separated in the clarifier is recycled upstream. In someembodiments from about 10% to about 20% of the recycled sludge isdirected to the wastewater inlet and contact tank through a conduit 275and from about 80% to about 90% of the recycled sludge is directed intothe biological treatment unit 230 through a conduit 265.

Utilizing DAF units as described above in a wastewater treatment systemprovides several advantages over similar wastewater treatment systemsoperated without DAF units. Because the DAF units remove a significantportion of suspended solids from influent wastewater without the needfor oxidation of these solids, the size of other components of thesystem may be reduced, resulting in a lower capital cost for the system.For example, primary clarifiers may be omitted from the wastewatertreatment system. Due to the reduced amount of oxidized solids to beremoved from the system, a final clarifier, such as the clarifier 140,may be reduced in size, in some embodiments by about 50%. Because alower amount of BOD enters the biological treatment unit (for example,the biological treatment unit 130), the size of the biological treatmentunit may be reduced, in some embodiments by about 30%. There is also alesser requirement for oxygen in the biological treatment unit whichallows for the capacity and power requirements of an aeration system inthe biological treatment unit to also be reduced, in some embodiments byabout 30%. The reduced size of the components of the treatment systemprovides for a decreased footprint of the system. For example, awastewater treatment plant with a capacity to treat 35 million gallonsper day (MGD) of wastewater with an influent BOD of 200 mg/L wouldrequire about 150,000 ft² of treatment units with a conventional designapproach; with embodiments of the present invention the footprint couldbe reduced to about 75,000 ft².

In other embodiments of systems and methods in accordance with thepresent invention, a wastewater treatment system, such as any of thosedescribed above, may further include an anaerobic treatment unit (ananaerobic digester). Non-limiting examples of components or portions ofanaerobic systems that can be utilized in one or more configurations ofthe wastewater treatment systems include, but are not limited to, theDYSTOR® digester gas holder system, the CROWN® disintegration system,the PEARTH® digester gas mixing system, the PFT® spiral guided digestergas holder, the PFT® vertical guided digester holder, the DUO-DECK™floating digester cover, and the PFT® heater and heat exchanger system,from Evoqua Water Technologies.

The anaerobic digester may be utilized to treat mixed liquor, which mayinclude suspended solids, sludge, and/or solids-rich or solids-leanfluid streams, from one or more other treatment units of the wastewatertreatment system. At least a portion of an anaerobically treated sludgeproduced in the anaerobic digester may be recycled back to one or moreother treatment units of the wastewater treatment system. The nature andfunction of the anaerobic digester and associated recycle streams may besimilar to those described in U.S. Pat. No. 8,894,856, titled “Hybridaerobic and anaerobic wastewater and sludge treatment systems andmethods,” which is herein incorporated by reference in its entirety forall purposes.

The systems and components of embodiments of the invention may providecost advantages relative to other wastewater treatment systems throughthe use of biological treatment processes in combination with anaerobicdigestion. The wastewater treatment systems and processes of embodimentsof the present invention can reduce sludge production through the use ofvarious unit operations including aerobic and anaerobic biologicalprocesses and recycle streams. The wastewater treatment processes alsoovercome some of the technical difficulties associated with use of someanaerobic wastewater treatment processes, by, for example, concentratingor strengthening the sludge introduced into the anaerobic digester.Additionally, costs associated with use of a conventional aerobicstabilization unit are typically reduced because less aeration wouldtypically be required in the aerobic processes due to the use of theanaerobic digester and various recycle streams. The various processescan also generate methane as a product of the anaerobic digestionprocess, which can be used as an energy source. In certain embodiments,a large portion of the chemical oxygen demand (COD) and BOD present ininfluent wastewater to be treated can be reduced using the anaerobicdigester. This can reduce the aeration and oxygen requirements, andthus, operation costs of the wastewater treatment system, and increasethe amount of methane produced that can be used as an energy source.Additionally, because anaerobic digestion can be used to reduce COD andBOD in the sludge, the sludge yield can also be reduced. The reductionof COD and/or BOD in the anaerobic treatment unit may also provide for areduction in size of the stabilization tank or other aerobic treatmentunit in the wastewater treatment system as compared to systems notutilizing the anaerobic digester.

Embodiments of the present invention may provide for the recirculationof aerobic bacteria, anaerobic bacteria, or both through various unitoperations of the treatment system.

It was previously believed that methanogens were strict anaerobicbacteria that would die quickly in an aerobic environment. Variousaspects of the invention, however, involve treatment systems andsubsystems, unit operations, and components thereof that accommodate orincrease the survivability of methanogenic organisms. One advantageousfeature of the treatment systems of the present application involvesproviding a large amount of methanogens through the anaerobic recycle toa contact stabilization process through the unique internal anaerobicsludge recycle path. At least a portion of the methanogenic bacteriareturn to the anaerobic digester, thereby seeding the anaerobic digesterwith methanogenic bacteria to join the existing population of the viablemethanogens in the anaerobic digester. This reduces the need for theanaerobic digester to have a size and resultant hydraulic residence timeor solids retention time to maintain a stable methanogenic bacteriapopulation in the absence of bacterial seeding, as in previously knownprocesses.

The concentration of seeding methanogenic bacteria, on a basis of acount of microorganisms, provided at the input of the anaerobic digestermay in some embodiments be at least a target percentage, such as about10% or more, of the concentration of the methanogenic bacteria presentin the anaerobically digested sludge stream exiting the anaerobicdigester. In some embodiments, this percentage may be, for example,about 25% or more, about 33% or more, about 50% or more, or about 75% ormore.

The anaerobic digester of systems in accordance with the presentinvention may be sized smaller than those in previously known systems.The methanogenic bacterial seeding of the anaerobic digester alsoprovides for a safety factor against disruptions of the anaerobicdigestion process. In the event of anaerobic digestion process upset orfailure, the anaerobic digesters of the presently disclosed systemswould recover faster than that the anaerobic digesters in previouslyknown systems because the seeding of the anaerobic digester withmethanogenic bacteria would add to the rate of replenishment ofmethanogenic bacteria in the anaerobic reactor due to the growth ofthese bacteria therein, reducing the time required for the anaerobicdigester to achieve a desired concentration of methanogenic bacteria.

The advantage of methanogen recycle can be estimated as follow:

$\theta_{x} = \frac{X_{a}V}{{QX}_{a} - {QX}_{a}^{0}}$

Where

-   -   V=Volume of anaerobic digester    -   θ_(x)=Solids retention time in anaerobic digester (days)    -   X_(a)=concentration of methanogens    -   Q=influent and effluent flow rate    -   X_(a) ⁰=concentration of methanogens in the inlet stream, which        is normally considered zero for conventional activated sludge        process.

If about 50% of methanogens survive in the short solid retention timecontact stabilization process and are recycled back to anaerobicdigester, the solids retention time of the anaerobic digester could bedoubled, or the size of the anaerobic digester decreased by half. Forexample, in previously known systems a hydraulic retention time in ananaerobic digester was in many instances set at between about 20 andabout 30 days. With a treatment system operating in accordance someembodiments of the present application, this hydraulic retention timemay be reduced by about 50% to between about 10 and about 15 days.

In some embodiments of the apparatus and methods disclosed herein, ahydraulic retention time in a treatment system contact stabilizationvessel may be about one hour or less. A significant portion ofmethanogens can be recycled in the short solid retention time contactstabilization aerobic process, which can reduce the capital cost andoperational cost of the anaerobic digester(s). For example, the tankvolume of the anaerobic digester(s) could be decreased to bring thesafety factor to a range closer to those anaerobic digester(s) without amethanogen recycle process. With smaller volume, the capital cost of theanaerobic digesters and the mixing energy consumption of the anaerobicdigestion process would both decrease, which will make apparatus andprocesses in accordance with the present disclosure more cost effectivethan previously known apparatus and processes.

In other embodiments, the seeding of the anaerobic digester withrecycled methanogenic bacteria may provide for decreasing the hydraulicresidence time of sludge treated in the digester. This would result in adecreased cycle time, and thus an increased treatment capacity of thetreatment system. Increasing the amount of methanogens recycled to theanaerobic digester, by, for example, increasing an amount ofmethanogen-containing sludge directed into the digester, would providegreater opportunity to decrease the hydraulic residence time in thedigester and increase the treatment capacity of the system.

If a significant portion of methanogens can be recycled in the aerobiccontact stabilization process, the capital cost and operational cost ofthe anaerobic digesters could be decreased. For example, the tank volumeof the anaerobic digesters could be decreased to bring the safety factorto a range closer to those anaerobic digesters in systems not includinga methanogen recycle process. With smaller volume, the capital cost ofthe anaerobic digesters and the mixing energy consumption of theanaerobic digesters will both decrease, which will make the wastewatertreatment process more cost effective.

In certain embodiments, the contact tank is constantly seeded withnitrification bacteria (such as ammonia oxidizing and nitrite oxidizingbiomass) which can survive the anaerobic digester and which can berecycled back to the aerobic environment. For example, nitrification andde-nitrification can take place in the contact tank. Nitrification maybe carried out by two groups of slow-growing autotrophs:ammonium-oxidizing bacteria (AOB), which convert ammonia to nitrite, andnitrite-oxidizing bacteria (NOB), which oxidize nitrite to nitrate. Bothare slow growers and strict aerobes. In some embodiments of treatmentsystems disclosed herein, the nitrification bacteria are introduced toand/or grown in a contact tank, where they are captured in the floc.Some of the nitrification bacteria will pass out from the contact tankand be sent to an anaerobic digester.

It was previously believed that the strictly anaerobic conditions of theanaerobic digester would kill the nitrification bacteria. Variousaspects of the invention, however, involve treatment systems andsubsystems, unit operations, and components thereof that accommodate orincrease the survivability of nitrification organisms in anaerobic andanoxic conditions that may occur in some biological nutrient removalprocesses. Nitrification bacteria which survive the anaerobic digesterand are returned to the aerobic part of the treatment process mayenhance the nitrification process performance in ways that can lowercapital costs, for example by providing for a reduced aerobic treatmentvessel size and/or reduced aerobic treatment hydraulic retention timeand/or an increased safety factor that would render the nitrificationprocess more stable in response to disruptions to the treatment process.Disruptions to the treatment process encompass deviations from desiredoperating parameters which may be caused by, for example, interruptionsin flow of material through the treatment system or a loss oftemperature control at one or more unit operations. The survival rate ofnitrification bacteria in an anaerobic digester could be increased bydecreasing a hydraulic residence time in the anaerobic digester, whichwould be accomplished if the anaerobic digester were seeded withrecycled methanogens, as described above.

A wastewater treatment system, indicated generally at 400 in FIG. 4,includes an anaerobic treatment unit 490, referred to herein as ananaerobic digester. The wastewater treatment system of FIG. 4 includes acontact tank 410, a DAF unit 420, a stabilization tank 430, a clarifier440, and associated fluid conduits 414, 424, 435, 445, 455, 465, and 475which are similar in structure and function to the contact tank 110, DAFunit 120, biological treatment unit 130, clarifier 140, and associatedfluid conduits 114, 124, 135, 145, 155, 165, and 175 of the systemillustrated in FIG. 1 and described above. A singular DAF unit 420 isillustrated in FIG. 4, although in alternate embodiments the treatmentsystem may use multiple DAF units as described above with reference tothe treatment system of FIG. 1.

In the system of FIG. 4, wastewater from a source of wastewater 405 isdirected into a primary clarifier 412 through an inlet of the primaryclarifier. A solids-rich fluid stream from the clarifier is directedthrough conduit 404 into an inlet of a thickener 480, which maycomprise, for example, a gravity belt thickener. A solids-lean effluentfrom the primary clarifier 412 is directed into an inlet of the contacttank 410 through conduit 402. A solids-rich output stream from thethickener 480 is directed to an inlet of the anaerobic digester 490through conduit 484. A solids-lean effluent from the thickener isdirected to an inlet of the contact tank 410 through conduit 482. Theanaerobic digester is also supplied with suspended solids removed frommixed liquor in the DAF unit 420 through conduits 425 and 484.

In some embodiments, a portion of the suspended solids removed from themixed liquor in the DAF unit 420 may be recycled to the contact tank 410through conduits 425 and 426 in a similar manner as the recycle ofsuspended solids removed in the DAF unit(s) 120 to the contact tank 110described above with reference to FIG. 1.

The solids-rich output stream from the thickener 480 and any suspendedsolids from the DAF unit 420 introduced into the anaerobic digester 490are combined and anaerobically digested in the anaerobic digester. Theanaerobic digestion process can be operated at temperatures betweenabout 20° C. and about 75° C., depending on the types of bacteriautilized during digestion. For example, use of mesophilic bacteriatypically requires operating temperatures of between about 20° C. andabout 45° C., while thermophilic bacteria typically require operatingtemperatures of between about 50° C. and about 75° C. In certainembodiments, the operating temperature may be between about 25° C. andabout 35° C. to promote mesophilic activity rather than thermophilicactivity. Depending on the other operating parameters, the retentiontime in the anaerobic digester can be between about seven and about 50days retention time, and in some embodiments, between about 15 and about30 days retention time. In certain embodiments, anaerobic digestion ofmixed liquor in the anaerobic digester may result in a reduction inoxygen demand of the mixed liquor of about 50%.

A first portion of an anaerobically digested sludge produced in theanaerobic digester may be recycled through an outlet of the anaerobicdigester and into the stabilization tank 430 through conduit 492. Thisrecycle stream may facilitate retaining sufficient solids in the systemto provide a desired residence time in the stabilization tank. Theanaerobically digested sludge recycled to the stabilization tank mayalso seed the stabilization tank with nitrification bacteria to enhancethe nitrification activity within the stabilization tank as describedabove. The anaerobically digested sludge recycled into the stabilizationtank may also contain methanogenic bacteria which are subsequentlyreturned to the anaerobic digester to enhance the performance of theanaerobic digester as described above.

In embodiments where the stabilization tank 430 includes an aeratedanoxic region and an aerobic region, such as in the biological treatmentunit 130 of FIG. 2 described above, the portion of the anaerobicallydigested sludge recycled to the stabilization tank may be directed intothe aerated anoxic region of the stabilization tank. A second portion ofthe anaerobically digested sludge produced in the anaerobic digester maybe sent out of the system as waste solids through a conduit 495. Thefirst portion of the anaerobically digested sludge recycled into thestabilization tank 430 may be any amount between about 0% and about 100%of the anaerobically digested sludge produced in and output from theanaerobic digester, with the second portion, making up the balance, sentout of the system as waste solids through conduit 495. In someembodiments, between about 0% and about 80% of the anaerobicallydigested sludge is recycled from one or more outlets of the anaerobicdigester to one or more other unit operations of the treatment system.

In another embodiment of the wastewater treatment system, indicatedgenerally at 500 in FIG. 5, the first portion of the anaerobicallydigested sludge produced in the anaerobic digester is recycled throughan outlet of the anaerobic digester and into the inlet of the contacttank 410 through conduit 494, rather than into the stabilization tank430. This recycle stream may facilitate providing sufficient activatedsludge in the contact tank to adsorb/absorb or enmesh BOD present in theinfluent wastewater. The anaerobically digested sludge recycled to thecontact tank may also seed the contact tank with nitrification bacteriato enhance the nitrification activity within the contact tank asdescribed above. The anaerobically digested sludge recycled into thecontact tank may also contain methanogenic bacteria which aresubsequently returned to the anaerobic digester to enhance theperformance of the anaerobic digester as described above. The firstportion of the anaerobically digested sludge recycled into the contacttank 410 may be any amount between about 0% and about 100% of theanaerobically digested sludge produced in and output from the anaerobicdigester, with a second portion, making up the balance, sent out of thesystem as waste solids through conduit 495.

In another embodiment of the wastewater treatment system, indicatedgenerally at 600 in FIG. 6, a first portion of the anaerobicallydigested sludge produced in the anaerobic digester may be recycledthrough an outlet of the anaerobic digester and into the inlet of thecontact tank 410 through conduit 494, and a second portion of theanaerobically digested sludge may be recycled through an outlet of theanaerobic digester and into the stabilization tank 430 through conduit492. These recycle streams may provide the benefits described above withregard to systems 400 and 500. A third portion of the anaerobicallydigested sludge may be directed to waste through conduit 495. The sum ofthe first portion of the anaerobically digested sludge and the secondportion of the anaerobic sludge may be any amount between about 0% andabout 100% of the anaerobically digested sludge produced in and outputfrom the anaerobic digester, with the third portion, making up thebalance, sent out of the system as waste solids through conduit 495. Therecycled anaerobic sludge may be split in any desired ratio between thefirst portion and the second portion. The first portion may comprisefrom about 0% to about 100% of all the anaerobically digested sludgeproduced in and output from the anaerobic digester with the sum of thesecond portion and the third portion making up the balance.

Another embodiment of the wastewater treatment system, indicatedgenerally at 700 in FIG. 7, is similar to that illustrated in FIG. 6,however the thickener 480 is not utilized. Rather, the solids-rich fluidstream from the clarifier is directed through conduit 406 into an inletof the DAF unit 420. The DAF unit 420 of the system illustrated in FIG.7 performs the function of the thickener 480 of the system illustratedin FIG. 6. The utilization of the DAF unit 420 to perform the functionof the thickener may reduce or eliminate the need for a thickener in thesystem, which may reduce both capital and operational costs of thesystem. A first portion of the anaerobically digested sludge created inthe anaerobic digester 490 is recycled to the contact tank 410 and asecond portion is recycled to the stabilization tank 430 to provide thebenefits described above. A third portion of the anaerobically digestedsludge is directed to waste through conduit 495.

Further embodiments may include any combination of features of thesystems described above. For example, in some embodiments, a firstportion of the solids-rich fluid stream from the clarifier is directedthrough conduit 406 into an inlet of the DAF unit 420, while a secondportion is directed into a thickener 480. In any of the aboveembodiments, the stabilization tank 430 may include an aerated anoxicregion and an aerobic region. A first portion of the anaerobicallydigested sludge recycled to the stabilization tank may be directed intothe aerated anoxic region of the stabilization tank and a second portionmay be recycled to the aerobic region. The ratio the amount of recycledanaerobic sludge directed to the aerated anoxic region to the amount ofrecycled anaerobic sludge directed to the aerobic region may be anyratio desired. Any of the embodiments disclosed herein may includemultiples of any of the treatment units and/or conduits illustrated.

In accordance with another embodiment, methods disclosed herein maycomprise directing the wastewater into an aeration unit for initialtreatment. The wastewater may be aerated with oxygen before anysuspended growth activated sludge treatment to form an aerated mixedliquor. Downstream, the methods may include directing the aerated mixedliquor into a contact tank for treatment, as previously described.

The aeration may promote microbial growth, increase TSS, and promoteadsorption/absorption of BOD, as previously described. The wastewatermay be provided with sufficient oxygen such that aerobic conditions aremaintained in at least a portion of the wastewater. Suspended anddissolved solids in the wastewater, including BOD, may become oxidized.The residence time of the wastewater in the aeration unit may besufficient for a significant amount of oxidation of the BOD to occur. Insome embodiments, for example, at least about 10% of the BOD is oxidizedprior to entering the contact tank. The residence time of the wastewaterin the aeration unit is in some embodiments from about 5 minutes toabout 90 minutes, and in some embodiments, from about 15 minutes toabout 45 minutes. The residence time may be adjusted depending uponfactors such as the BOD of the influent wastewater. A wastewater with ahigher BOD may require longer treatment in the aeration unit thanwastewater with a lower BOD.

In some embodiments, as previously described, the contact tank may beaerated. In certain embodiments, the contact tank is aerated more thanthe aeration unit. In other embodiments, the contact tank and theaeration unit are aerated at substantially the same rate. In otherembodiments, the contact tank is not aerated, such that most or allprimary aeration of the wastewater occurs upstream from the contacttank.

Aerating the wastewater to be treated prior to directing the wastewaterto the contact tank may provide one or more benefits, for example, morebiogas production, less energy consumption, and smaller systemfootprint. Briefly, by aerating the wastewater upstream from the contacttank, the amount of TSS in the contact tank may further be increased ascompared to methods not including an upstream aeration step. Aspreviously described, the increased TSS level in the contact tank mayprovide for additional soluble BOD to be adsorbed/absorbed in thecontact tank as compared to a contact tank having a lower level of TSS.In some embodiments, a desirable TSS level in the contact tank may bebetween about 1,200 mg/L and about 3,500 mg/L. Alternatively, certainmethods may enable soluble BOD to be adsorbed/absorbed in the aerationunit as compared to the contact tank. In such embodiments, the desirableTSS level in the contact tank may be less than 3,500 mg/L, for example,between about 600 mg/L and about 2,400 mg/L.

Aerating the wastewater upstream from the contact tank, for example, inthe aeration unit, may additionally or alternatively reduce necessaryvolume of the contact tank. In some embodiments, the contact tankpositioned downstream from an aeration unit may be at least about 70%smaller than a contact tank in a system without a primary aeration unit.The contact tank may be at least about 60%, at least about 50%, at leastabout 40%, at least about 30%, or at least about 20% smaller than acontact tank in a system without an upstream aeration unit.

In some embodiments, the wastewater to be treated may be high soluble,low particulate organic content wastewater. The wastewater may have ahigher solids content than municipal wastewater. For example, thewastewater may have a solids content of at least about 5%. Thewastewater may have a solids content of at least about 7%, at leastabout 10%, at least about 15%, or at least about 20%. For instance, themethods described herein may enable treatment of industrial wastewaterby dissolved air flotation. As disclosed herein, industrial wastewaterincludes wastewater associated with an industrial process or system. Thecharacteristics of industrial wastewater may generally be dependent onthe industry. In some embodiments, the industrial wastewater has morethan 300 mg/L or more than 500 mg/L TSS; more than 300 mg/L or more than350 mg/L BOD; more than 750 mg/L or more than 1000 mg/L COD; or morethan 750 mg/L or more than 1000 mg/L total dissolved solids (TDS).

The aeration unit may be a short hydraulic retention time (HRT) aerationchamber. Generally, aeration may include delivering an oxygen containinggas, for example, air, into a tank with the wastewater. The gas may bedispersed through the wastewater by one or more pumps.

In some embodiments, the aeration unit may provide upstream treatment ofthe wastewater prior to contacting the wastewater with activated sludge.In other embodiments, at least a portion of the activated sludge may berecycled to the aeration unit, to provide primary adsorption/absorptionof BOD in the wastewater. Suspended and dissolved solids in thewastewater, including BOD, may be adsorbed/absorbed into the activatedsludge in the aeration unit, forming a first mixed liquor. The residencetime of the wastewater in the aeration unit may be sufficient for themajority of the BOD to be adsorbed/absorbed by the activated sludge. Insome embodiments, the residence time of the wastewater in the aerationunit may be sufficient for more BOD to be adsorbed/absorbed in theaeration unit than downstream in the contact tank. The amount of solidsrecycled to the aeration unit may range from about 1% to about 100% of atotal amount of solids removed from the DAF unit(s). The amount ofactivated sludge recycled to the aeration unit may be a majority of atotal amount of activated sludge recycled, for example, greater thanabout 50%, between about 50% and about 95%, or between about 60% andabout 80% of the total amount of activated sludge recycled. In someembodiments, the amount of activated sludge recycled to the aerationunit is greater than the amount of activated sludge recycled to thecontact tank. The amount of activated sludge recycled to the aerationunit may be greater than the amount of activated sludge recycled to thecontact tank and to any other unit, for example, the biologicaltreatment unit.

In some embodiments, at least a portion of the suspended solids removedin the DAF unit(s) may be recycled to the aeration unit. Recyclingsolids removed in the DAF unit(s) to the aeration unit may provide forgreater oxidation of BOD in the aeration unit than in the contact tankbecause the solids recycled to the aeration unit include living bacteriacapable of oxidizing BOD. The amount of solids recycled to the aerationunit may range from about 1% to about 100% of a total amount of solidsremoved from the DAF unit(s). The amount of solids recycled to theaeration unit may be a majority of a total amount of solids removed fromthe DAF unit(s), for example, greater than about 50%, between about 50%and about 95%, or between about 60% and about 80% of the total amount ofsolids removed in the DAF unit(s). In some embodiments, the amount ofsolids recycled to the aeration unit is greater than the amount ofsolids recycled to the contact tank. The amount of solids recycled tothe aeration unit may be greater than the amount of solids recycled tothe contact tank and any other conduit, for example, an anaerobicdigester.

Another embodiment, indicated generally at 1300, is provided in FIG. 13.The system 1300 includes an aeration unit 670, a contact tank 610, a DAFunit 620, a stabilization tank 630 (biological treatment unit), andassociated fluid conduits 614, 624, 635, 625, and 626, which are similarin structure and function to elements of the systems illustrated inFIGS. 1 and 2. The system 1300 includes fluid conduit 628 extendingbetween the biological treatment unit 620 and the aeration unit 670(through fluid conduits 625 and 626). The system 1300 includes fluidconduit 674 extending between the aeration unit 670 and the contact tank610. Two DAF units 620 are illustrated in FIG. 13, although in alternateembodiments the treatment system may use a single DAF unit as shown, forexample, in the treatment system of FIG. 4.

In system 1300, wastewater from a source of wastewater 605 is directedinto an aeration unit 670 through an inlet of the aeration unit. Anoxygen containing gas from a source of gas 607 is directed into theaeration unit 670 through another inlet of the aeration unit. The oxygencontaining gas may treat the wastewater by aeration to form an aeratedmixed liquor. The aerated mixed liquor is directed into contact tank610. Downstream from contact tank 610, gas from a source of gas 608 isdirected into DAF unit 620 to float suspended solids. Furtherdownstream, the process liquid flows through stabilization tank 630, aspreviously described herein.

System 1300 includes controller 638 and metering valve 616 which areoperably connected to each other. Metering valve 616 may be positionedand configured to selectively direct floated solids from DAF unit 620 toaeration unit 670 (through conduits 625, 626, and 628) and contact tank610 (through conduits 625 and 626). Controller 638 may be configured toinstruct the metering valve 616 to selectively direct floated solids, aspreviously described.

Another embodiment indicated generally at 1400 is shown in FIG. 14.System 1400 is similar to system 1300 but further includes a clarifier640 (solids-liquid separation unit) and associated fluid conduits 645,655, 665, and 675, which are similar in structure and function toelements of the system illustrated in FIG. 1. System 1400 includes fluidconduit 685 extending between the solids-liquid separation unit 640 andthe aeration unit 670. System 1400 includes metering valve 618 which isoperably connected to controller 638. Controller 638, shown in FIG. 14,is operably connected to metering valves 616 and 618, but in certainembodiments separate controllers may be provided for each meteringvalve. Metering valve 618 may be positioned and configured toselectively direct activated sludge from solids-liquid separation unit640 to aeration unit 670 (through conduit 685) and contact tank 610(through conduit 675). Controller 638 may be configured to instruct themetering valve 618 to selectively direct activated sludge, as previouslydescribed.

The controller 638 may be programmed to operate metering valves 616 and618 automatically, for example, on a schedule or responsive to ameasurement or calculation received or determined by the controller 638.For example, in some embodiments, the controller 638 may obtain ameasurement associated with the composition of the wastewater. Thecontroller 638 may operate one or more metering valve (616 and/or 618)to adjust treatment of the wastewater in the aeration unit 670 orcontact tank 610. The controller 638 may operate one or more pump toadjust treatment of the wastewater in the system. The measurement may beinput manually or obtained from a sensor operably connected to thecontroller 638. Any controllers, sensors, metering valves, or pumpsknown to one of ordinary skill in the art may be provided to operate asdescribed herein.

Another embodiment indicated generally at 1500, is illustrated in FIG.15. In this embodiment, the wastewater treatment system 1500 is brokeninto two separate but interconnected subsystems, one subsystem 1500Aincluding aeration unit 670, contact tank 610 and DAF unit(s) 620, and asecond subsystem 1500B including biological treatment unit 630 andsolids-liquid separation unit 640. As shown in FIG. 15, aeration unit670 and contact tank 610 are two separate treatment tanks, although insome embodiments the units may be included in the same treatment tank.As shown in FIG. 15, in the first subsystem 1500A influent wastewaterfrom a source of wastewater 605A is directed into the aeration unit 670.In the second subsystem 1500B influent wastewater from a source ofwastewater 605B is directed into the biological treatment unit 630. Thesubsystems of FIG. 15 are similar in structure and function to elementsof the subsystems illustrated in FIG. 3.

Stabilization tank 630 is shown in FIGS. 13-15, although in alternateembodiments the biological treatment unit 630 may include an aerobicregion and an aerated anoxic region, as shown in FIG. 2. The systems ofFIGS. 13-15 may also include an anaerobic digester as shown in FIG. 4.Additionally, in some embodiments, wastewater from the source ofwastewater 605 may be directed to the contact tank 610, as shown in FIG.1.

EXAMPLES Example 1

A wastewater treatment system 1000 was configured as illustrated in FIG.10, where the indicated unit operations and conduits have the samestructure and function as the identically indicated unit operations andconduits in FIGS. 4-7. The wastewater treatment system 1000 was used toexamine the effects of recycling removed solids from the DAF unit 420 tothe contact tank 410. By gradually increasing the amount of removedsolids from the DAF unit 420 recycled to the contact tank 410 from 0% ofthe solids removed in the DAF unit to about 90% of the solids removed inthe DAF unit over the course of three weeks, the suspended solids (MLSS)content of contact tank was brought up from 600 mg/L to over 1200 mg/L.The DAF dissolved solids content increased from 3%-4% prior to beginningthe recycle of solids from the DAF unit to the contact tank to above 5%after beginning the recycle of solids from the DAF unit to the contacttank. The total suspended solids (TSS) removal efficiency of the DAFunit increased from about 75% to over 85%. The COD removal of the DAFunit increased from about 70% to about 80% over the course of thetesting. These results are illustrated in the charts of FIG. 11 and FIG.12.

These results show that recycling removed solids from a DAF unit to acontact tank in a system such as that illustrated in FIG. 10 may providefor a greater amount of suspended solids in the contact tank. Theincreased amount of suspended solids in the contact tank increases theamount of suspended and soluble COD and BOD which may be removed fromwastewater influent to the contact tank and absorbed/adsorbed/enmeshedin the suspended solids and/or which may be oxidized in the contacttank. Recycling removed solids from a DAF unit to a contact tank in asystem such as that illustrated in FIG. 10 increases the efficiency ofthe removal of suspended solids in the DAF unit. These effects maydecrease the load on downstream unit operations and may reduce operatingcosts of the system as a whole and/or may reduce capital costs of thesystem by providing for smaller downstream processing units to beutilized. Further, a greater amount soluble BOD/COD from wastewaterinfluent to the system may be removed as solids in the DAF unit and maybe sent from the DAF unit to an anaerobic digester instead of an aerobictreatment unit operation, reducing the aeration power requirements ofthe system and increasing the amount of biogas that could be produced.

Prophetic Example 1

In this prophetic example, a water treatment system was configured asillustrated in FIG. 1 with the biological treatment unit 130 comprisinga single tank.

Assumptions of Feed:

The system was fed wastewater at a rate of 57,600 gallons/day (gpd), 40gallons per minute (gpm). The wastewater was assumed to be typical ofmunicipal wastewater, having a total BOD (tBOD) of 140 mg/l (67 lbs/day)of which 43% (60 mg/l, 29 lbs/day) was particulate (non-soluble) BOD(pBOD), and 57% (80 mg/l, 38 lbs/day) was soluble BOD (sBOD). Thewastewater was also assumed to include 100 mg/l (48 lbs/day) ofsuspended solids (SS), of which 19 lbs/day (48 lbs/day SS−29 lbs/daypBOD) was assumed to be inert (non-biological) material, and 6 lbs/dayof ammonia.

HDT Assumptions:

The hydraulic detention time (HDT) in the contact tank 110 was assumedto be 45 minutes and the hydraulic detention time (HDT) in thebiological treatment unit 130 was assumed to be five hours.

Flow Rate Through Contact Tank:

The ratio of return sludge sent from the clarifier 140 to the contacttank was set at 2.4 lb/lb of tBOD, for a (2.4)(67 lbs/day tBOD)=160lbs/day recycled sludge or 2,880 gpd (2.0 gpm), assuming a recycledsludge solids loading of 6,660 mg/l. The total flow through the contacttank was thus 57,600 gpd+2,880 gpd=60,480 gpd (42 gpm).

From laboratory bench scale testing, it was found that in the contacttank, approximately 50% of the sBOD was removed, with approximately ⅔ ofthe amount removed converted to SS, and approximately ⅓ of the amountremoved oxidized, for example, converted to carbon dioxide and water.Thus, it was assumed that in the contact tank 14 lbs/day of sBOD wasconverted to SS and 5 lbs/day of pBOD was oxidized. The total solidspassed through the contact tank was thus 160 lbs/day recycled sludge+48lbs/day suspended solids from influent wastewater+14 lbs/day sBODconverted to SS−5 lbs pBOD oxidized=217 lbs/day. The mixed liquorsuspended solids (MLSS) leaving the contact tank was thus ((217lbs/day)/(60,480 gpd))(453592.4 mg/lb)(0.2641721 gal/l)=430 mg/l.

The tBOD leaving the contact tank was 67 lbs/day input−5 lbs/dayoxidized=62 lbs/day (121 mg/l). The sBOD leaving the contact tank was 38lbs/day in−14 lbs/day converted to SS−5 lbs/day oxidized=19 lbs/day (37mg/l). The pBOD leaving the contact tank was 29 lbs/day influent+14lbs/day converted from sBOD=43 lbs/day (84 mg/l).

Flow Split into DAF and Biological Treatment Tank:

The flow out of the contact tank was split between the DAF units 120 andthe biological treatment unit 130. 46.5% (101 lbs/day, 28,080 gpd, 19.5gpm) of the output of the contact tank was directed to the DAF units and53.5% (116 lbs/day, 32,400 gpd, 22.5 gpm) was directed into thebiological treatment unit.

It was assumed that all recycled sludge directed to the DAF units (160lbs/day introduced into contact tank−116 lbs/day returned to biologicaltreatment tank=44 lbs/day) was removed in the DAF process.

BOD Influent to Biological Treatment Unit:

The total BOD to be treated in the biological treatment unit includesthe BOD from the contact tank (53.5% of 62 lbs/day=33 lbs/day) in 32,400gpd of influent plus BOD from the DAF units. The pBOD influent to theDAF units was 46.5% of 43 lbs/day output from contact tank=20 lbs/day.The sBOD influent to the DAF units was 46.5% of 19 lbs/day output fromthe contact tank=9 lbs/day at a flow rate of 28,800 gpd. Assuming 80% ofthe pBOD was removed in the DAF units, the tBOD flowing from the DAFunits to the biological treatment tank was (0.2*20 lbs/day pBOD)+9lbs/day sBOD=13 lbs/day tBOD. Thus the total influent BOD to thebiological treatment tank was 33 lbs/day from the contact tank+13lbs/day from the DAF units=46 lbs/day.

Solids in Biological Treatment Tank:

The biological treatment unit was sized to accommodate a BOD loading of29 lbs/1000 ft³, a common loading in the industry. This meant that thevolume of the biological treatment unit was (46 lbs/day influenttBOD)/(29 lbs/1000 ft³ tBOD loading)=1,600 ft³ (12,000 gal). This volumeresulted in a HDT in the biological treatment unit of (12,000 gal/57,600gpd)(24 hr/day)=5 hours. The total solids in the biological treatmentunit was set at 220 lbs, for a total MLSS of (220 lbs/12,000 gal)(0.264gal/l)(453,592 mg/lb)=2200 mg/l. Assuming a sludge yield of 95% of theBOD results in an amount of waste sludge produced in the biologicaltreatment unit of (0.95)(46 lbs/day tBOD)=44 lbs/day waste sludge. Thewaste sludge age would thus be (220 lbs total solids)/(44 lbs/day wastesludge)=5.2 days.

Biological Treatment Tank Oxygen Requirements:

It was assumed that 0.98 lbs of oxygen were required to oxidize a poundof BOD and 4.6 lbs of oxygen were required to oxidize a pound ofammonia. The oxygen requirement of the biological treatment unit wasthus (0.98 lbs O₂/lb BOD)(46 lbs tBOD/day)+(4.6 lbs O₂/lb ammonia)(6lb/day ammonia)=72.6 lb/day O₂ (3 lb O₂/hr). Using a FCF (FieldCorrection Factor—a correction factor to compensate for the reducedoxygen absorbing ability of mixed sludge in the biological treatmenttank as opposed to clean water) of 0.5, this results in a specificoxygen utilization rate (SOUR) of 6 lbs O₂/hr. Assuming diffused air wassupplied to the biological treatment tank from a aeration systemsubmerged by nine feet and a 6% oxygen transfer capability (OTE), thebiological treatment unit would require a flow of (6 lbsO₂/hr)(1/0.06)(1/60 hour/min)(1/1.4291 l/g O₂)(453.6 g/lb)(0.035ft³/l)=18.5 ft³/min (scfm), or if aerating with air with approximately20% O₂, 92.6 scfm.

Clarifier:

The clarifier was assumed to have a 61 ft³ volume. 57,600 gpd flowedinto the clarifier, resulting in an overflow of 57,600 gpd/61 ft³=944gallon per ft³ per day (gpcfd) overflow rate. Assuming an MLSS of 2200mg/l from the biological treatment tank and targeting a recycled sludge(RAS) concentration of 6600 mg/l and 50% of overflow recycled as RASgives a RAS flow rate of 20 gpm (28,800 gpd). It was assumed that 18 gpmRAS was recycled to the biological treatment tank and 2 gpm to thecontact tank. The solids loading of the clarifier was thus (57,600 gpdinfluent wastewater+28,800 gpd RAS)(2200 mg/l MLSS)(1/453592.4lb/mg)(3.79 l/gal)/(61 ft²)=(1588 lbs/day)/(61 ft²)=26 lb/ft²·day.

Solids Wasted:

Solids wasted in DAF units: 101 lbs/day (assuming 100% efficiency).

Ratio of sludge wasted to BOD treated: (101 lbs/day)/(67 lbs/day tBOD inwastewater influent)=1.5

With the addition of the DAF units to the treatment system in the aboveexample, the amount of tBOD to be treated in the biological treatmenttank was reduced from 62 lbs/day to 46 lbs/day, a reduction of 26%. Thisprovided for a reduced required size for the biological treatment tankto obtain a desired solids loading and resulted in a decrease in therequired amount of air needed to treat this tBOD in the biologicaltreatment tank. This would translate into a cost savings for bothcapital costs, for a reduced size of the biological treatment tank andaeration system, as well as a decreased operating cost due to thereduced amount of aeration required.

Prophetic Example 2

A simulation was performed using BIOWIN™ simulation software (EnviroSimAssociates Ltd., Ontario, Canada) to compare the performance of awastewater treatment system in accordance with an embodiment of thepresent invention with and without an anaerobic sludge recycle.

The wastewater treatment system without the anaerobic sludge recycleincluded was configured as illustrated in FIG. 8, indicated generally at800. This system is similar to that illustrated in FIG. 4, but with noanaerobic sludge recycle conduit 492 and with the addition of a membranebioreactor (MBR) 510 which receives a solids lean effluent from theclarifier 440 through conduit 442. The MBR produces a product waterpermeate which is removed from the system through conduit 445, and asolids-rich retentate, which is recycled to the DAF unit 480 throughconduit 444. The MBR 510 was simulated to perform complete nitrificationof the solids lean effluent from the clarifier 440.

The performance of the wastewater treatment of FIG. 8 was simulated andcompared to the simulated performance of the wastewater treatment system900 of FIG. 9. Wastewater treatment system 900 of FIG. 9 is similar towastewater treatment system 800 of FIG. 8, but with the addition of ananaerobic sludge recycle conduit 492 recycling anaerobically digestedsludge from the anaerobic digester 490 to the stabilization tank 430though conduit 492. In the simulation of the wastewater treatment system900, 45% of the anaerobically digested sludge output from the anaerobicdigester 490 was recycled to the stabilization tank 430, and 55% of theanaerobically digested sludge output from the anaerobic digester 490 wassent to waste.

The simulation of the performance of both systems 800 and 900 assumed aninfluent wastewater flow rate of 100 MGD. The influent wastewater wasassumed to have a COD of 500 mg/L, a total suspended solids (TSS) of 240mg/L, a Total Kjeldahl Nitrogen (TKN) of 40 mg/L, and a temperature of15° C.

The results of the simulation indicated that the anaerobically digestedsludge recycle of the system 900 resulted in a decrease in the totaloxygen requirement for treating the influent wastewater as compared tothe system 800 of from 113,891 kg O₂/day to 102,724 kg O₂/day, a savingsof about 10%. Assuming an oxygen transfer energy requirement of 1.5 kgO₂/kwh, this reduction in oxygen consumption would reduce the powerrequirements associated with providing the oxygen from 75,988 kwh/day to68,483 kwh/day, a savings of 7,515 kwh/day.

The results of the simulation indicated that the anaerobically digestedsludge recycle of the system 900 resulted in an increase in the amountof methane produced as compared to the system 800 from 1,348 scfm to1,652 scfm, an increase of about 23%. Assuming that 35% of the methanechemical energy could be converted to electricity, the potentialelectricity generation from the methane produced would increase from104,511 kwh/day to 128,989 kwh/day.

Combining the energy reduction from the reduced oxygen requirement withthe energy gain from the increased methane production results in anenergy savings of about 31,982 kwh/day for the system 900 including theanaerobically digested sludge recycle as compared to the system 800without the anaerobically digested sludge recycle.

The results of the simulation also indicated that adding theanaerobically digested sludge recycle of the system 900 to the system800 resulted in a reduction in biomass (sludge) production from 81,003pounds per day to 61,167 pounds per day, a reduction of about 25%.

This simulation data indicates that the addition of an anaerobicallydigested sludge recycle to wastewater treatment systems in accordancewith the present invention may result in a significant reduction inpower consumption and a significant decrease in waste sludge production,both of which would result in cost savings and enhancedenvironmental-friendliness of the wastewater treatment system.

Prophetic Example 3

Calculations were performed to compare the performance of a wastewatertreatment system in accordance with an embodiment of the presentinvention with and without a recycle of solids removed in a DAF unit ofthe system to a contact tank of the system. The wastewater treatmentsystem was configured as illustrated in FIG. 10.

It was assumed that the system was provided with 40 million gallons perday of wastewater influent with a BOD level of 250 mg/L (83,400 lbs/day)and suspended solids of 252 mg/L (84,000 lbs/day).

It was assumed that the biological treatment tank 430 operated with asolids retention time (SRT) of 5 days, a mixed liquor suspended solids(MLSS) concentration of 3,000 mg/L and a BOD loading of 45 lbs/1,000cubic feet (20.4 kg/28.3 cubic meters) and that all solids separated inthe clarifier 440 were recycled to the contact tank 410. The hydraulicdetention time (HDT) of the contact tank 410 was assumed to be 25minutes for the system operating without the DAF to contact tank solidsrecycle and one hour for the system operating with the DAF to contacttank solids recycle. The increase in HDT in the contact tank for thesystem when operating with the DAF to contact tank solids recycle was toprovide for the increased MLSS in the contact tank to adsorb additionalsoluble BOD in the contact tank as compared to the system operatingwithout the DAF to contact tank solids recycle. For the system operatingwith a recycle of solids from the DAF unit to the contact tank, it wasassumed that the DAF unit removed 308,000 lbs/day (139,706 kg/day) ofsolids from the mixed liquor passing through it and recycled 190,000lbs/day (86,183 kg/day, 62% of the solids removed) to the contact tankwhile directing 118,000 lbs/day (53,524 kg/day) of solids to theanaerobic digester 490.

A comparison of the results of the calculations comparing the systemwith and without the DAF to contact tank solids recycle is illustratedin Table 1 below:

TABLE 1 System operated without DAF → System with 62% DAF → ParameterContact tank recycle Contact tank recycle BOD treated in biological41,200 (18,688 kg/day) 20,600 (9,344 kg/day) treatment tank (lbs/day)Aeration energy (both 600 410 contact tank and biological treatmenttank, kW) Solids to anaerobic digester 103,000 (46,720 kg/day) 115,000(52,163 kg/day) (lbs/day) Solids destroyed (lbs/day) 43,900 (19,913kg/day) 55,900 (25,356 kg/day) Biogas produced (mcfd/day) 0.66 (18,633cubic meters/day) 0.84 (23,730 cubic meters/day) Biogas energy (assuming40% 1,880 2,390 conversion efficiency, kW) Net energy gain (kW) 1,2801,880

These results show that providing a wastewater treatment system asconfigured in FIG. 10 with a recycle of solids removed in a DAF unit toa contact tank can significantly reduce the energy required to operatethe system as compared to an equivalent system without the recycle ofsolids from the DAF unit to the contact tank. Adding the DAF to contacttank solids recycle results in less BOD being sent for treatment in thebiological treatment tank (a reduction of (41,200−20,600)/41,200=50% inthe present example) which lowers the need for aeration in thebiological contact tank. A greater amount of biogas((0.84−0.66)/0.66=27% more in the present example) is produced whenadding the DAF to contact tank solids recycle to the system. Thecombined gain in biogas production and decrease in aeration energyrequirements results in a net energy gain of 1,880−1,280=600 kW whenadding the DAF to contact tank solids recycle to the system. At anestimated $0.10/kW energy cost, this net energy gain would yield a costsavings of about $530,000 per year.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

What is claimed is:
 1. A wastewater treatment system comprising: anaeration unit having a first inlet fluidly connectable to a source of awastewater to be treated, a second inlet fluidly connectable to a sourceof oxygen, a third inlet, and an outlet, the aeration unit configured toaerate the wastewater with the oxygen to form a first mixed liquor; acontact tank having a first inlet fluidly connected to the outlet of theaeration unit, a second inlet, and an outlet, the contact tankconfigured to treat the first mixed liquor with activated sludge to forma second mixed liquor; a dissolved air flotation unit having a firstinlet fluidly connected to the outlet of the contact tank, a secondinlet fluidly connected to a source of gas, a first floated solidsoutlet fluidly connected to the third inlet of the aeration unit, asecond floated solids outlet fluidly connected to the second inlet ofthe contact tank, and an effluent outlet, the dissolved air flotationunit configured to treat the second mixed liquor with the gas to formfloated solids and effluent; and a biological treatment unit having afirst inlet fluidly connected to the effluent outlet of the dissolvedair flotation unit, and an outlet, the biological treatment unitconfigured to treat the effluent from the dissolved air flotation unitto form a third mixed liquor.
 2. The system of claim 1, furthercomprising: a first metering valve positioned upstream from the firstfloated solids outlet and the second floated solids outlet; and a firstcontroller operatively connected to the first metering valve andconfigured to instruct the first metering valve to selectively dispensethe floated solids to the aeration unit and the contact tank.
 3. Thesystem of claim 1, wherein the contact tank has a third inlet and thebiological treatment unit has a second inlet, the system furthercomprising: a solids-liquid separation unit having an inlet fluidlyconnected to the outlet of the biological treatment unit, a solids-leaneffluent outlet, a first return activated sludge outlet fluidlyconnected to the third inlet of the contact tank, and a second returnactivated sludge outlet fluidly connected to the second inlet of thebiological treatment unit, the solids-liquid separation unit configuredto treat solids from the third mixed liquor to form solids-lean effluentand return activated sludge.
 4. The system of claim 3, wherein thesolids-liquid separation unit has a third return activated sludge outletand the aeration unit has a fourth inlet fluidly connected to the thirdreturn activated sludge outlet.
 5. The system of claim 4, furthercomprising: a second metering valve positioned upstream from the firstreturn activated sludge outlet and the third return activated sludgeoutlet; and a second controller operatively connected to the secondmetering valve and configured to instruct the second metering valve toselectively dispense the return activated sludge to the contact tank andthe aeration unit.
 6. The system of claim 1, wherein the contact tankhas a third inlet fluidly connectable to the source of the wastewater tobe treated.
 7. The system of claim 1, wherein the dissolved airflotation unit has a third floated solids outlet, the system furthercomprising an anaerobic digester having an inlet fluidly connected tothe third floated solids outlet.
 8. The system of claim 1, comprising afirst subsystem including the aeration unit, contact tank, and dissolvedair flotation unit and a second subsystem including the biologicaltreatment unit and the solids-liquid separation unit.
 9. The system ofclaim 8, wherein the aeration unit and the contact tank are included ina same treatment tank.
 10. A method of treating wastewater comprising:directing the wastewater into an aeration unit and aerating thewastewater with oxygen to form an aerated mixed liquor; directing theaerated mixed liquor into a contact tank and mixing the aerated mixedliquor with an activated sludge to form an activated mixed liquor;directing the activated mixed liquor into a dissolved air flotation unitand separating the activated mixed liquor to form a floated biosolidsand an effluent; selectively directing a first portion of the floatedbiosolids into the aeration unit and a second portion of the floated biosolids into the contact tank; and directing the effluent into abiological treatment unit and biologically treating the effluent to forma biologically treated mixed liquor.
 11. The method of claim 10, furthercomprising: directing the biologically treated mixed liquor into asolids-liquid separation unit and separating the biologically treatedmixed liquor to form a solids-lean effluent and the activated sludge;and directing a first portion of the activated sludge into thebiological treatment unit.
 12. The method of claim 11, furthercomprising selectively directing a second portion of the activatedsludge into the contact tank and a third portion of the activated sludgeinto the aeration unit.
 13. The method of claim 10, further comprisingdirecting a second portion of the wastewater into the contact tank. 14.The method of claim 10, comprising directing the wastewater having atleast about 5% solids content into the aeration unit.
 15. The method ofclaim 14, comprising directing the wastewater having at least about 20%solids content into the aeration unit.
 16. The method of claim 10,wherein a minority fraction of biological oxygen demand in thewastewater introduced into the aeration unit is oxidized in the aerationunit.
 17. The method of claim 16, wherein a greater amount of biologicaloxygen demand in the wastewater introduced into the aeration unit isoxidized in the aeration unit than biological oxygen demand in theaerated mixed liquor is oxidized in the contact tank.
 18. A method ofretrofitting a wastewater treatment system comprising a contact tank, adissolved air flotation unit, and a biological treatment unit, themethod comprising: providing an aeration unit having a first inletfluidly connectable to a source of wastewater to be treated, a secondinlet fluidly connectable to a source of oxygen, a third inlet, and anoutlet, the aeration unit configured to aerate the wastewater with theoxygen to form an aerated mixed liquor; fluidly connecting the outlet ofthe aeration unit to an inlet of the contact tank; fluidly connectingthe third inlet to a floated solids outlet of the dissolved airflotation unit; fluidly connecting a first metering valve positioneddownstream from the floated solids outlet; and comprising installing afirst controller operatively connected to the first metering valve andconfigured to instruct the first metering valve to selectively dispensethe floated solids to the aeration unit and the contact tank.
 19. Themethod of claim 18, wherein the system further comprises a solids-liquidseparation unit, the method further comprising fluidly connecting afourth inlet of the aeration unit to a return activated sludge outlet ofthe solids-liquid separation unit.
 20. The method of claim 19, whereinthe system further comprises a second metering valve positioneddownstream from the return activated sludge outlet, the methodcomprising installing a second controller operatively connected to thesecond metering valve and configured to instruct the second meteringvalve to selectively dispense the return activated sludge to theaeration unit and the contact tank.